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<strong>IMPORTANT</strong>: No additional bug fixes or documentation updates
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<a href="../current/index.html">current release documentation</a>.
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<span class="breadcrumb-node">Translog</span>
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<div class="chapter">
<div class="titlepage"><div><div>
<h2 class="title">
<a id="index-modules-translog"></a>Translog<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/index-modules/translog.asciidoc">edit</a>
</h2>
</div></div></div>
<p>Changes to Lucene are only persisted to disk during a Lucene commit, which is a
relatively expensive operation and so cannot be performed after every index or
delete operation. Changes that happen after one commit and before another will
be removed from the index by Lucene in the event of process exit or hardware
failure.</p>
<p>Lucene commits are too expensive to perform on every individual change, so each
shard copy also writes operations into its <em>transaction log</em> known as the
<em>translog</em>. All index and delete operations are written to the translog after
being processed by the internal Lucene index but before they are acknowledged.
In the event of a crash, recent operations that have been acknowledged but not
yet included in the last Lucene commit are instead recovered from the translog
when the shard recovers.</p>
<p>An Elasticsearch <a class="xref" href="indices-flush.html" title="Flush API">flush</a> is the process of performing a Lucene commit and
starting a new translog generation. Flushes are performed automatically in the
background in order to make sure the translog does not grow too large, which
would make replaying its operations take a considerable amount of time during
recovery. The ability to perform a flush manually is also exposed through an
API, although this is rarely needed.</p>
<h3>
<a id="_translog_settings"></a>Translog settings<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/index-modules/translog.asciidoc">edit</a>
</h3>
<p>The data in the translog is only persisted to disk when the translog is
<code class="literal">fsync</code>ed and committed. In the event of a hardware failure or an operating
system crash or a JVM crash or a shard failure, any data written since the
previous translog commit will be lost.</p>
<p>By default, <code class="literal">index.translog.durability</code> is set to <code class="literal">request</code> meaning that
Elasticsearch will only report success of an index, delete, update, or bulk
request to the client after the translog has been successfully <code class="literal">fsync</code>ed and
committed on the primary and on every allocated replica. If
<code class="literal">index.translog.durability</code> is set to <code class="literal">async</code> then Elasticsearch <code class="literal">fsync</code>s and
commits the translog only every <code class="literal">index.translog.sync_interval</code> which means that
any operations that were performed just before a crash may be lost when the node
recovers.</p>
<p>The following <a class="xref" href="indices-update-settings.html" title="Update index settings API">dynamically updatable</a> per-index
settings control the behaviour of the translog:</p>
<div class="variablelist">
<dl class="variablelist">
<dt>
<span class="term">
<code class="literal">index.translog.sync_interval</code>
</span>
</dt>
<dd>
How often the translog is <code class="literal">fsync</code>ed to disk and committed, regardless of
write operations. Defaults to <code class="literal">5s</code>. Values less than <code class="literal">100ms</code> are not allowed.
</dd>
<dt>
<span class="term">
<code class="literal">index.translog.durability</code>
</span>
</dt>
<dd>
<p>Whether or not to <code class="literal">fsync</code> and commit the translog after every index, delete,
update, or bulk request. This setting accepts the following parameters:</p>
<div class="variablelist">
<dl class="variablelist">
<dt>
<span class="term">
<code class="literal">request</code>
</span>
</dt>
<dd>
(default) <code class="literal">fsync</code> and commit after every request. In the event of hardware
failure, all acknowledged writes will already have been committed to disk.
</dd>
<dt>
<span class="term">
<code class="literal">async</code>
</span>
</dt>
<dd>
<code class="literal">fsync</code> and commit in the background every <code class="literal">sync_interval</code>. In
the event of a failure, all acknowledged writes since the last
automatic commit will be discarded.
</dd>
</dl>
</div>
</dd>
<dt>
<span class="term">
<code class="literal">index.translog.flush_threshold_size</code>
</span>
</dt>
<dd>
The translog stores all operations that are not yet safely persisted in Lucene
(i.e., are not part of a Lucene commit point). Although these operations are
available for reads, they will need to be replayed if the shard was stopped
and had to be recovered. This setting controls the maximum total size of these
operations, to prevent recoveries from taking too long. Once the maximum size
has been reached a flush will happen, generating a new Lucene commit point.
Defaults to <code class="literal">512mb</code>.
</dd>
</dl>
</div>
<h4>
<a id="index-modules-translog-retention"></a>Translog retention<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/index-modules/translog.asciidoc">edit</a>
</h4>
<div class="warning admon">
<div class="icon"></div>
<div class="admon_content">
<h3>Deprecated in 7.4.0.</h3>
<p>Translog retention settings are deprecated in favor of <a class="xref" href="index-modules-history-retention.html" title="History retention">soft deletes</a>. These settings are effectively ignored since 7.4 and will be removed in a future version.</p>
</div>
</div>
<p>If an index is not using <a class="xref" href="index-modules-history-retention.html" title="History retention">soft deletes</a> to
retain historical operations then Elasticsearch recovers each replica shard by replaying
operations from the primary’s translog. This means it is important for the
primary to preserve extra operations in its translog in case it needs to
rebuild a replica. Moreover it is important for each replica to preserve extra
operations in its translog in case it is promoted to primary and then needs to
rebuild its own replicas in turn. The following settings control how much
translog is retained for peer recoveries.</p>
<div class="variablelist">
<dl class="variablelist">
<dt>
<span class="term">
<code class="literal">index.translog.retention.size</code>
</span>
</dt>
<dd>
This controls the total size of translog files to keep for each shard.
Keeping more translog files increases the chance of performing an operation
based sync when recovering a replica. If the translog files are not
sufficient, replica recovery will fall back to a file based sync. Defaults to
<code class="literal">512mb</code>. This setting is ignored, and should not be set, if soft deletes are
enabled. Soft deletes are enabled by default in indices created in Elasticsearch
versions 7.0.0 and later.
</dd>
<dt>
<span class="term">
<code class="literal">index.translog.retention.age</code>
</span>
</dt>
<dd>
This controls the maximum duration for which translog files are kept by each
shard. Keeping more translog files increases the chance of performing an
operation based sync when recovering replicas. If the translog files are not
sufficient, replica recovery will fall back to a file based sync. Defaults to
<code class="literal">12h</code>. This setting is ignored, and should not be set, if soft deletes are
enabled. Soft deletes are enabled by default in indices created in Elasticsearch
versions 7.0.0 and later.
</dd>
</dl>
</div>
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